1. Field of the Invention
This invention relates to apparatus and method for the coulometric metering of the state of charge of a battery, for use, for instance, in a handheld telecommunications device.
2. Description of the Prior Art
Conventionally, battery state of charge meters depend on either measuring the voltage across the battery, or measuring the battery current and charging or discharging a battery modeling device. Judging battery state of charge purely on the basis of the battery voltage is difficult because of variations of battery voltage with load and temperature. Battery history is also relevant. Furthermore, a high level of accuracy of voltage measurement is required as the voltage fluctuates little.
In so-called coulometric metering using a battery modeling device, the modeling device is charged and discharged in accordance with the current flowing into and out of the battery. The modeling device has traditionally been an analog device, but a digital model is more appropriate for use in handheld telecommunications devices. Periodically sampling the battery current and converting the current samples to digital format for digital processing involves various problems. To keep the power consumed by the metering process as low as possible, the sampling frequency should kept be low, but this can give rise to inaccuracies caused by short current pulses between samples. The accuracy of conversion also needs to be high and, because very low current levels are involved, small errors introduced by the analog components of the sampling circuitry can become significant.